Embodiments of the disclosure relate generally to methods and systems for manufacturing of composite structures, and more specifically, to methods and systems for controlling the temperature of a workpiece during a direct manufacturing process.
Direct manufacturing processes such as selective laser sintering (SLS) are used for producing parts and other freeform solid articles by building layers of material into the finished product. SLS is used to form such articles by sintering a powder together in a predetermined pattern representing a cross section of the article at a plurality of elevations. As used herein sintering is defined as a process by which particulates are made to form a solid mass through the application of external energy. During a SLS build process, the external energy is focused and controlled by controlling a laser to sinter selected locations of a heat-fusible powder. By performing this process layer-by-layer, complex parts and freeform solid articles which cannot be fabricated easily (if at all) by subtractive methods such as machining can be quickly and accurately fabricated. Accordingly, SLS is particularly beneficial in the production of prototype parts, and is particularly useful in the customized manufacture of such parts and articles directly from computer-aided-design data bases.
SLS is performed by depositing a layer of a heat-fusible powder onto a target surface; examples of the types of powders include metal powders, polymer powders such as wax that can be subsequently used in investment casting, ceramic powders, and plastics such as ABS plastic, polyvinyl chloride (PVC), polycarbonate, and other polymers. Portions of the layer of powder corresponding to a cross-sectional layer of the part to be produced are exposed to a focused and directionally controlled energy beam, such as generated by a laser having its direction controlled by mirrors, under the control of a computer. The portions of the powder exposed to the laser energy are sintered into a solid mass in the manner described hereinabove. After the selected portions of the layer have been so sintered or bonded, another layer of powder is placed over the layer previously selectively sintered, and the energy beam is directed to sinter portions of the new layer according to the next cross-sectional layer of the part to be produced. The sintering of each layer not only forms a solid mass within the layer, but also sinters each layer to previously sintered powder underlying the newly sintered portion.
However, known SLS systems typically experience a high incidence of warpage and shrinkage of the article due to thermal effects. Such warpage may result in the curling of a sintered layer such that the layer does not bond to an adjacent previously sintered layer. In cases where the layers of the part bond together, the part itself may warp. In many cases warpage may be caused by thermal shrinkage of the sintered layer during a transition from a sintering temperature to a temperature approaching the temperature of the workpiece area. Moreover, uneven cooling of the part during manufacture such that upper layers of the part are cooled more quickly than lower layers may contribute to warpage and curling.
Accordingly, accurate control of the temperature of the article being produced may facilitate reducing such warpage. At least some known systems use convective heating methods, which because of inconsistency in the flow of heated air in the workpiece area does not permit accurate control of the temperature of the article. Additionally, radiant heaters such as floodlamps, quartz rods, and conventional flat radiant panels placed near the target surface have been used to attempt to control the temperature of the part being produced. However, accurate control of temperature requires the expertise of expensive, educated personnel to manage the production process on each machine which significantly increases operating costs. Such variability experienced in both the mechanical and chemical properties of the part and in the process is a barrier preventing the transition of SLS direct manufacturing into a mainstream production process. Additionally, the currently used heater, a two zone, foil in ceramic element is limited to relatively low temperatures because a difference in the coefficient of thermal expansion (CTE) for the two materials causes the heater to self destruct at temperatures needed for improved process performance.
What are needed are methods and systems for providing sufficient energy in a spatially accurate workpiece area that can be controlled between different heating zones such that a differential temperature between different portions of an article being manufactured may be controlled to a predetermined value.